This invention pertains to methods of testing materials, including methods of testing sputtering target materials. The sputtering target materials can be tested before the materials have acquired a sputtering target form (for instance, as-cast materials), and methods of testing the materials after they have acquired a sputtering target form (with sputtering target form referring to microscopic features, such as, for example, grain size and orientation), including methods of testing the materials after they have been processed into a sputtering target shape. In particular applications, the invention pertains to non-destructive evaluation of sputtering target materials through the use of ultrasonic testing technologies.
One method of applying thin films of materials during manufacturing of integrated electronic circuits is sputtering of the materials from a target. The sputtering comprises forming a target of a material which is to be deposited, and providing the target as a negatively charged cathode proximate a strong electric field. The electric field is utilized to ionize a low pressure inert gas and thereby form a plasma. Positively charged ions in the plasma are accelerated by the electric field toward the negatively charged cathode (i.e., toward the sputtering target). The ions impact the sputtering target, and thereby eject target material. The ejected target material is primarily in the form of atoms or groups of atoms, and can be utilized to deposit thin, uniform films on substrates placed in the vicinity of the target during the sputtering.
It is frequently desired to manufacture sputtering targets to tight tolerances to ensure uniformity in the thickness and conformity of layers formed by sputtering materials from the targets. A problem that can occur during the manufacture of sputtering targets is formation of discontinuities (i.e. heterogeneous regions) in sputter target material. Exemplary discontinuities are voids, cracks and changes in porosity throughout a material, as well as solid inclusions. The sputter target discontinuities can lead to unipolar arcing during a sputtering process. Unipolar arcing can cause localized overheating and explosion of target material, and can thus decrease the uniformity and conformity of thin film deposition on a substrate. Additionally, if the discontinuities are inclusions or other regions comprising impurities, sputtering from the discontinuities can result in particles of the discontinuities being deposited onto a substrate.
As integrated circuit devices become increasingly smaller, tolerances for uniformity, conformity and undesired particles decrease. Accordingly, it is desired to form better (i.e. more homogeneous) target materials, and also desired to develop testing technologies which can distinguish homogeneous target materials from inhomogeneous target materials.
Among the techniques which can be most useful for testing target materials are techniques which are non-destructive to the materials. In other words, techniques which evaluate the materials but which still enable the materials to be utilized in formation and utilization as sputtering targets. A non-destructive technique which has received recent interest is ultrasonic testing. For instance, ultrasonic testing technologies are described in PCT International Application Number PCT/US99/13066 and U.S. Pat. No. 5,887,481. General principles of ultrasonic testing technology are also described in a Non-Destructive Testing Handbook (A. S. Birks, R. E. Green, P. McIntire, Non-Destructive Testing Handbook, Ultrasonic Testing, Second Edition, Vol. 7, American Society for Non-Destructive Testing, 1991.)
The Non-Destructive Testing Handbook describes modem C-scan ultrasonic testing equipment. Such equipment uses an automated X-Y raster scan of an ultrasonic transducer relative to a tested material. The transducer scans parallel, and in close proximity to a surface of the sample, with the sample and transducer both being coupled through a sound conducting medium, such as, for example, water, oil, or a gel. At each point in the scan, a pulse of ultrasound is transmitted from the ultrasonic transducer, through the sound-conducting medium, and into the sample. The pulse travels from the transducer as an xe2x80x9cultrasonic beamxe2x80x9d. A fraction of the ultrasonic beam can be reflected, refracted or scattered back to the transducer from discontinuities in the path of the beam. The ultrasound signals returned to the transducer are converted into electrical signals. The electrical signals are processed so that only signals returned from discontinuities located within a specified depth range below the sample surface are evaluated. For each point in the raster scan, the amplitude and time is recorded for the largest amplitude signal that corresponds with the specified depth range. The amplitude and time data for all scan points comprise a so-called findamental data set. The amplitude data are often coded by color or shade and plotted versus scan position in a xe2x80x9cC-scan image,xe2x80x9d such as, for example, the image shown in FIG. 1. The image of FIG. 1 shows four xe2x80x9cdefectsxe2x80x9d (actually flat-bottom holes) in a target material, with the defects labeled as xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d, xe2x80x9cCxe2x80x9d and xe2x80x9cDxe2x80x9d.
Compensation for various depth effects can be achieved by applying a multiplicative correction factor to the C-scan amplitude. The depth correction can be applied during analog signal processing (for example, utilizing a time varied gain), or during digital processing, (for example, utilizing depth amplitude correction). The depth correction factor can be derived from measurements of flat bottom holes of a specified diameter. The flat bottom holes are drilled various depths into the back surface of a standard, which is measured using ultrasound incident on the front surface. The front and back surfaces of the standard are parallel, and the holes are normal to these surfaces. The distance from the front surface to the blind-end of a flat bottom hole is considered the depth of the flat bottom hole with respect to ultrasound testing. Depth amplitude correction factors are derived for depths corresponding to the available flat bottom hole depths, and interpolated or extrapolated for other depths. The size of an unknown defect can be estimated by comparison to flat bottom hole reference standards.
U.S. Pat. No. 5,887,481 and PCT Application Number PCT/US99/13066 describe particular applications of ultrasound testing technology to the testing of target materials. For instance, U.S. Pat. No. 5,887,481 describes a method of sorting aluminum target materials into several grades based on the number of defects per cubic centimeter detected by ultrasonic testing technology. One potential drawback of U.S. Pat. No. 5,887,481 is that the estimation of a defect size is generally based on only a single (highest amplitude) datapoint. Depending on the relative position of the defect and the raster scan points, differences in maximum amplitude are expected. PCT Application No. PCT/US99/13066 discusses U.S. Pat. No. 5,887,481 in detail, and provides techniques to overcome perceived shortcomings in U.S. Pat. No. 5,887,481. For instance, PCT/US99/13066 describes a method which avoids the units of defect per cubic centimeter, and rather presents defect information as a histogram. Instead of estimating the size of individual defects, PCT/US99/13066 analyzes the amplitude distribution of all datapoints in a C-scan ultrasonic testing type analysis. Any datapoint in the analysis having an amplitude which exceeds a threshold is considered a defect datapoint. The threshold is determined based on measurements of flat bottom hole reference standards and target material samples. A so-called xe2x80x9ccleanlinessxe2x80x9d factor is calculated as a ratio of defect datapoints (referred to in PCT/US99/13066 as xe2x80x9cflaw datapointsxe2x80x9d) to total datapoints. Alternatively, a histogram may be generated by counting the number of datapoints which fall into various xe2x80x9camplitude bands.xe2x80x9d
While U.S. Pat. No. 5,887,481 and PCT/US99/13066 present advances in methods for analyzing target materials, there are difficulties associated with the techniques. It would be desirable to develop improved techniques for utilizing ultrasound in determining homogeneity of target materials.
In one aspect, the invention encompasses a method for testing materials, such as for example sputtering target materials, other materials for electronics applications, or other materials in general in which homogeneity throughout the material is desired. A plurality of positions are defined across at least a portion of a material. Sonic energy is sequentially irradiated across the plurality of positions. Echoes are induced by the sonic energy, and detected. At least some of the detected echoes are associated with individual positions of the plurality of positions that triggered the detected echoes. Information pertaining to at least one physical attribute of the detected echoes is processed to sort the detected echoes into a first group indicative of inhomogeneities in the material, and a second group which does not indicate inhomogeneities in the material. The echoes of the first group are sorted into clusters, with the clusters being defined as echoes from adjacent positions of the plurality of positions. The separate echoes associated with a common cluster are analyzed and considered together to generate information about an inhomogeneity in the material.